Prior to the present invention, it was generally known that aluminosilicate materials, for example, mullite, a naturally occurring, high temperature performing material composed essentially of chemically combined aluminum, oxygen and silicon atoms x(Al.sub.2 O.sub.3)SiO.sub.2, where x is 1.5 to 2.0, was an attractive candidate for a variety of high temperature applications. In addition, mullite, unlike closely related aluminum-oxygen-silicon materials, is known to be highly resistant to attack by strong acids and other corrosive reagents, as taught by R. F. Davis and T. A. Pask, "Mullite", pp. 37-76 in "High Temperature Oxides", Part IV, Allen M. Alper, ed., Academic Press, NY (1971). However, no technique was known for making mullite in a form such as a high temperature, corrosion resistant, coating. It would be highly desirable, therefore, to provide a procedure whereby mullite could be made synthetically in an appropriate form to utilize its outstanding properties.
As shown by W.A.D.C. Technical Report, 58-160 ASTIA document No. 155675, The Air Force Inorganic Polymer Program, R. L. Rau (June 1958), Pages 21-25, silicon-oxygen-aluminum polymers can be made by effecting reaction between an aluminum chelate dialkoxide, for example, diisopropoxyaluminum acetylacetonate and a difunctional silane, such as dimethylacetoxysilane. Reaction was carried out in boiling toluene to produce a variety of products varying from soft resins, waxes, or powders. It has been found that the aforementioned aluminum-oxygen-silicon materials of R. L. Rau provide glass-like coatings when heated at temperatures exceeding 350.degree. C. in an oxidizing atmosphere, for example air. However, the resulting aluminosilicate coatings fall outside of the mullite compositions range, and do not provide optimum coating characteristics on ceramic or metal substrates in particular applications. Additional procedures for making organoaluminosilanes are shown by S. N. Varisov et al, Organosilicon Heteropolymers and Heterocompounds, Plenum Press, New York (1970). However, none of these procedures lead to the preparation of organoaluminosilanes with Al/Si atomic ratios in the range appropriate for mullite.
H. Dislich, New Routes to Multicomponent Oxide Glasses, Angewandte Chemie, International Edition Vol. 10, pages 383-434 (1971) has described the preparation of coherent multicomponent oxide glass coatings on various substrates using mixtures of metal alkoxides in organic solvents. Similarly, Yoldas and Partlow, Formation of Continuous Beta Alumina Films and Coatings at Low Temperatures, Ceramic Bulletin, Vol. 59, No. 6, (1980) pages 640-642, describe the preparation of continuous films of NaAl.sub.11 O.sub.17 on ceramic substrates using solutions of the corresponding metal alkoxides. In both reports, removal of the organic component is effected by hydrolysis of the organometallic film after deposition and the resultant metal oxide films do not possess the desired thermal and chemical stability characteristic of mullite.
As shown by K. S. Mazdiyasni et al, Synthesis and Mechanical Properties of Stoichiometric Aluminum Silicate (Mullite), Pages 548,552, Vol. 55, No. 11, Journal of the American Ceramic Society, a method for preparing mullite is provided by reacting aluminum triisopropoxide and silicon tetrakisisopropoxide under reflux conditions in isopropyl alcohol. The resulting alkoxide solution can be ammoniated to produce the corresponding hydroxy aluminosilicate which can be dried in vacuum to produce mullite powders. However, the aforementioned technique was unsuitable for applying a mullite coating onto various substrates.
Improved results have been obtained as shown by U.S. Pat. No. 4,434,103, Interrante, assigned to the same assignee as the present invention, when chelated aluminum alkoxide of the formula ##STR1## where R is a polyvalent organic radical, Y is selected from a C.sub.(1-13) monovalent hydrocarbon radical and substituted C.sub.(1-13) monovalent hydrocarbon radical, and Z is selected from --O--, --S-- and --N.dbd., were coreacted with difunctional organosilane to produce an organic substituted silicon-oxygen-aluminum oligomer referred to hereinafter as the "organoaluminosiloxane". The organoaluminosiloxane was applied onto a temperature resistant substrate as an organic solvent solution which was transformed into an aluminosilicate coating upon heating. Although the aforementioned method of Interrante provided valuable aluminosilicate coatings on various substrates, it was often not possible to obtain a precise control over the Al/Si ratio in the organoaluminosiloxane or the resulting aluminosilicate coating derived therefrom. This resulted in an aluminosilicate coating providing a reduced degree of corrosion protection. The degree of corrosion protection provided by an aluminosilicate coating can be determined by the gain in weight of an aluminosilicate coated sample subjected to a sodium sulfite-sulfur trioxide atmosphere at 750.degree. C. In addition, the use of a difunctional silane as a coreactant with the chelated aluminum alkoxide of formula (1) requires an initial prehydrolysis of such chelated aluminum alkoxide before the hydrolysis product can be coreacted with the difunctional organosilane.
The present invention is based on the discovery that organosilane having the formula, EQU (R.sup.1).sub.a SiX.sub.4-a, (2)
where R.sup.1 is selected from Y radicals as previously defined and "a" has a value of about 2.5 to 3 inclusive, can be directly coreacted with a chelated aluminum alkoxide of formula (1) in the presence of an organic solvent to produce an organic solvent soluble organoaluminosiloxane having a predictable Al/Si ratio such as 3 to 1. In addition, the production of such organoaluminosiloxane in accordance with the practice of the present invention does not require any prehydrolysis of the chelated aluminum alkoxide of formula (1) as required in the method of Interrante, U.S. Pat. No. 4,434,103.
Although it is not completely understood why a predictable Al/Si ratio is feasible in the organoaluminosiloxane made in accordance with the practice of the present invention, and not in accordance with the method of U.S. Pat. No. 4,434,103, one possible explanation is that the use of difunctional silane results in the formation of cyclopolydiorganosiloxane which readily separates from the organoaluminosiloxane coating upon pyrolysis during the formation of the aluminosilicate coating.